"Function" type for java-created enumerators with size. Should be implemented so that calls to SizeFn#size are kept in sync with the size of the created enum (i.e. if the object underlying an enumerator changes, calls to SizeFn#size should reflect that change).
size(org.jruby.runtime.ThreadContext context, T self, org.jruby.runtime.builtin.IRubyObject[] args)